Reconstruction of Holocene wildfire occurrence using levoglucosan and lignin biomarkers from Siberian stalagmites

Recent accelerating global temperature rise increases the likelihood and susceptibility of the Siberian taiga to more frequent and extreme wildfires [1] [2]. This leads to enhanced permafrost thaw and subsequent greenhouse gases emissions, in a positive feedback loop [3]. Various studies have examined these paleofires in Siberia on limited, modern timescales [4, 5], but long-term reconstructions of wildfire occurrences are scarce [6]. This study reconstructs wildfire occurrence during the Holocene using stalagmites from southern Siberia. We provide a new means for assessing Siberian wildfires during interglacial periods and the first southern Siberian Holocene wildfire record. 

Three stalagmites from Botovskaya Cave (55˚17’59”N, 105˚19’46”E) have been U/Th-dated at the Oxford geochronology laboratory. These speleothem samples were collected deep inside the poorly ventilated cave, which is overlain by 40-130 m of sandstone covered by a thin soil and boreal taiga forest. Drip sites are active year-round, and cave air temperature is stable at ca. 1.3±0.5°C. Wildfires sporadically occur above the cave.

We use novel speleothem biomarkers, levoglucosan and lignin, as tracers for wildfire activity and vegetation composition above the cave, respectively. Levoglucosan is an anhydrous monosaccharide solely produced by the combustion of cellulose, and thus an ideal proxy for wildfires. Lignin is a biopolymer with three monomers. The monomer ratio can inform on relative changes between gymnosperm vs. angiosperm plant communities. Using both proxies we can decipher not only wildfire recurrence, but also changes in vegetation (e.g., from pine forest to peatbogs or grassland).

We took subsamples between 300 and 1000 mg and attribute the levels of levoglucosan to variance of the composition of lignin monomers, corresponding with vegetation composition. The required sample size (1 g) and the low observed carbonate growth rates of ca. 4-8 mm/a mean that we can only achieve multi-centennial resolution for the Holocene. To gain complementary insights into environmental conditions we combine the biomarker information with stable isotopes and element concentrations.

 References 

[1] V. I. Kharuk, et al., "Wildfires in the Siberian Taiga," Ambio , vol. 50, pp. 1953-1974, 2021.

[2] M. B. S. Flannigan, et al., "Impact of climate change on fire acivity and fire management in the circumboreal forest," Global Change Biology, vol. 15, pp. 549-560, 2009.

[3] M. R. Turetsky, et al., "Carbon release through abrupt permafrost thaw," Nature Geoscience , vol. 13, pp. 138-143, 2020.

[4] M. M. Grieman, et al.,"Aromatic acids in a Eurasian Arctic ice core: a 2600-year proxy record of biomass burning," Climate of the Past , vol. 13, pp. 395-410, 2017.

[5] R. Glückler, et al., "Wildfire history of the boreal forest of south-western Yakutia (Siberia) over the last two millennia document by a lake-sediment charcoal record," Biogeosciences , vol. 18, pp. 4185-4209, 2021.

[6] E. Dietze, et al., "Relationships between low-temperature fires, climate and vegetation during three late glacials and interglacials of the last 430 kyr in northeastern Siberia reconstructed from monosaccharide anhydrides in Lake El'gygytygyn sediments," Climate of the Past, vol. 16, pp. 799-818, 2020.